The bonding of electrically conducting materials is generally accomplished using metal-containing solders. In their unaltered form, organic adhesives are not generally considered for creating an electrically conductive bond because although the bond provided exhibits high strength, it does not allow the flow of electrons across the bond. That is, unaltered organic adhesives act as insulators rather than conductors. Since the use of solders often requires temperatures that may be impracticable for a given environment, it is desirable to have an electrically conductive adhesive. For example, in some applications where a printed wiring board cannot withstand soldering temperatures it is necessary to use an adhesive. Also, when heat is applied to any metal it may lose its dimensional stability, therefore suggesting the use of adhesives for bonding in place of soldering or welding. EMI (electromagnetic interference) and RFI (radio-frequency interference) shields generally consist of metal pieces with gaskets at the joints to ensure minimal EMI/RFI leakage. Although these gaskets can be soldered or welded to the metal, the high temperatures required by these operations may reduce the metal compliancy and degrade the EMI/RFI performance. To overcome this problem adhesive bonding is often employed. In field repair work it is often easier to use an electroconductive adhesive than soldering or welding.
There are many types of electrically conductive adhesives and pastes available. These pastes or adhesives contain electrically conducting particles incorporated into the self-hardening organic adhesive. The particles, including carbon black, copper, silver, or other noble metals, are mixed with the non-conducting organic adhesive to facilitate the flow of electrical current through the adhesive bonding layer. It has been observed that a decrease in the electrical resistivity of the bonding layer is generally at the expense of the bond strength. To obtain good conductivity the electrically-conducting filler must be used in such high proportions with respect to the organic adhesive that either inadequate adhesion and/or a weak bonding layer results. None of the available electroconductive adhesives exhibit desirable high-strength and high-temperature characteristics. Due to the use of essentially homogeneous shapes for the conductive filler, the prior art electroconductive adhesives exhibit one or more of the following undesirable properties: excessive resistivity, inadequate shear strength, a change in the modulus (i.e., rigidity or stiffness) of the adhesive, and a change in the coefficient of thermal expansion of the adhesive.
One such electrically conductive adhesive is disclosed in U.S. Pat. No. 3,359,145. The composition consists essentially of a hardenable organic adhesive in the mobile state and from 5% to 60% by weight of a ferromagnetic electrically conducting, finely particulated filler. One interface of the adhesive layer and the item to be bonded is maintained normal to the lines of force of an external magnetic field while the adhesive is hardening to orient the particles of the filler toward the opposite interface.